differentiation of water-rich planetary bodies

nWhereas in the Inner Solar System, most planetary bodies differentiated dry (or water-poor) into a basaltic protocrust, a peridotite mantle and an iron-rich core, in the Outer Solar System, the building blocks for planetesimals were water-rich, producing ice-bearing bodies. The role and fate of water during the differentiation process, however, is still largely unconstrained. As the temperature increases due to the extinct radioactivities (e.g. 26Al) and to accretion energy, differentiation of icy-planetesimals follows three main steps:

(1) Ice melting produces a two-layer body, with a water ocean (potentially partially frozen) above a rocky core. At the same time, interaction between liquid water and rock produces a large amount of hydrous silicates, mainly serpentines.

(2) If temperature increases sufficiently, the rocky core undergoes metamorphism, and follows a series of dehydration reactions that progressively releases more water toward the ice layer.

(3) With further temperature increase, metal- then silicate-solidii are reached, and the rocky core can undergo further differentiation into a metallic core, a silicate mantle, and possibly a silicate crust.

Step (1) is mostly controlled by the melting curve of ice and is relatively well known. However, almost nothing is known about further differentiation steps, even if we know that some of the differentiated bodies in the outer solar system possess a metallic core (Io, Europa, Ganymede) and thus likely reached at least the metal melting curve. In this project, I use phase equilibrium diagrams of water-saturated Fe-rich compositions and thermal models for the evolution of small planetary bodies to discuss the influence of large amounts of water on planetary differentiation.